Many recent designs of electric powered and hybrid electric powered vehicles employ a regenerative braking system in order to increase operating efficiency. During a braking event, the electric motor which normally drives one or more traction wheels is switched to operate as an electrical generator. Using the momentum and kinetic energy of the vehicle, the electric drive motor generates electricity that may be used to recharge on-board energy storage systems, such as batteries and ultra capacitors, power accessories, or power auxiliary on-board systems.
Regenerative braking systems are particularly effective in recovering energy during city driving, where driving patterns of repeated acceleration and decelerations are common. Electric drive vehicles employing regenerate braking typically utilize traditional friction brakes, along with a vehicle control system that coordinates the operation of the friction brakes and the regenerative brake in order to provide adequate stopping ability while making dual brake operations essentially transparent to the driver. Normally, such a control system controls the electric motor torque to perform regenerative braking until the vehicle decelerates to a certain speed at which time the friction brakes are gradually applied to bring the vehicle to a compete stop.
The dual braking strategy described above may not be optimum for certain types of electric drive configurations, and may not be appropriate for configurations where it is desirable to completely avoid friction braking components. For example, a two axle vehicle might be provided with friction brakes on the wheels of only one axle; clearly it would be desirable to provide an electric means of fully braking the axle not equipped with friction brakes. In some configurations, it may be desirable to completely avoid the use of friction brakes, thus necessitating the use of some electronic means of achieving adequate braking. Even in those configurations where all wheels are equipped with friction brakes, it may be desirable to provide frictionless electric braking for each axle in the event that the friction brakes are intentionally or unintentionally disabled for any reason.
Accordingly, a need exists in the art for a braking system for vehicles with electric drive systems capable of providing frictionless deceleration and braking of the vehicle to all speeds down to and including zero speed, regardless of the configuration of the vehicle's motor drive, axles and wheels. The present invention is intended to satisfy this need.